1. Field
The present invention relates to high-efficiency light emitting diodes (LEDs).
2. Discussion of the Background
In general, since Group-III-element nitrides, such as gallium nitride (GaN) and aluminum nitride (AlN), have an excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as materials for visible and ultraviolet light emitting devices. Particularly, blue and green light emitting devices using indium gallium nitride (InGaN) are used in various applications, such as large-sized full-color flat panel displays, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, and optical communications.
Since it is difficult to fabricate a homogeneous growth substrate for Group-III-element nitride semiconductors, Group-III-element nitride semiconductor layers are grown on a heterogeneous substrate having a crystal structure similar to that of the Group-III-element nitride semiconductor, through processes such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). A sapphire substrate having a hexagonal grain structure is frequently used as the heterogeneous substrate. However, since sapphire is an electrical insulator, it limits the structure of a light emitting diode (LED) formed thereon. Accordingly, there has recently been developed a technique in which epitaxial layers, such as nitride semiconductor layers, are grown on a heterogeneous substrate such as sapphire, a support substrate is bonded to the epitaxial layers, and the heterogeneous substrate is then separated using a laser lift-off technique or the like, thereby fabricating a high-efficiency vertical-type LED (e.g., see U.S. Pat. No. 6,744,071).
FIG. 1 is a sectional view illustrating a conventional LED. Referring to FIG. 1, a conventional vertical-type LED is fabricated by sequentially forming a GaN-based n-type layer 23, a GaN-based active layer 25, and a GaN-based p-type layer 27 on a growth substrate (not shown), forming a p-electrode 39 having a reflective metal layer on the p-type layer 27, flip-bonding the p-electrode 39 to a Si submount 41 using a bonding metal 43, removing the growth substrate, and then forming an n-electrode 37 on the exposed n-type layer 23. An n-electrode 45 is then formed on the bottom surface of the Si submount 41. Furthermore, in U.S. Pat. No. 7,704,763, the surface of the exposed n-type layer 23 is roughened using a dry or photo-enhanced chemical (PEC) etching technique, thereby enhancing the light extraction efficiency.
In addition, the support substrate is generally a conductive substrate in such a conventional LED. Thus, the conventional LED has a vertical-type structure, in which the n-electrode and the p-electrode are disposed opposite to each other.
However, in the conventional LED, since only the upper surface of the n-type layer 23 is roughened, light loss occurs due to the internal total reflection generated at side surfaces of the semiconductor stack 20. Further, since the n-electrode 37 or an n-electrode pad is positioned on the n-type GaN layer, the light generated in the active layer can be absorbed or reflected by the n-electrode 37, decreasing the light extraction efficiency. In addition, Ag is frequently used to form a reflection layer that is in ohmic contact with the p-type GaN layer. However, the Ag may be easily aggregated during a thermal treatment process, which results in current leakage, due to the migration of Ag atoms, during operation of the LED. Therefore, it is difficult to form a stable reflection layer using Ag. Furthermore, Ag has reflectance limitations because it is a metallic material.